1. Field of the Invention
The present invention relates to a field effect transistor (FET) and particularly to a high electron mobility transistor (HEMT).
2. Description of the Prior Art
An HEMT is disclosed by T. Mimura et al. in Japanese Journal of Applied Physics Vol. 19, No. 5, May, 1980, pp. L225-L227. In this HEMT, a non-doped GaAs layer and an AlGaAs layer doped with Si are stacked on a semi-insulating GaAs substrate. The Si dopant is ionized by emitting electrons and acts as donor. The thus generated electrons are stored on the GaAs side of the heterogeneous interface between the AlGaAs layer and the GaAs layer because of a low energy level at the bottom of the conduction band of non-doped GaAs. The electrons thus stored two-dimensionally form a channel, which is controlled by gate voltage. In the HEMT, the non-doped GaAs channel layer and the carrier supply layer of AlGaAs doped with impurity are provided independently and, accordingly, even if density of carriers in the channel layer is increased by doping the carrier supply layer with a large amount of impurity, mobility of electrons is not lowered since the channel layer is not doped with impurity. Consequently, the HEMT is capable of performing high-speed operation.